A need exists in the art for improved polymeric materials that are highly permeable, yet which under certain circumstances, provide selective separation of the components of various gaseous mixtures. Such materials would be especially useful in commercial, non-cryogenic gas separation processes.
Commercial applications utilizing separation devices based on polymeric materials rely, in part, on maximizing the overall gas flux through the membrane. P. H. Kim, et al., J. Appl. Poly. Sci., 34 1761 (1987), reports that the gas flux for a membrane is related to the average space between the polymer chains. In addition, the reference states that overall gas flux is related to the density of the polymer. The success of potential commercial applications depends on the identification of polymers which possess very high flux and good thermo-mechanical properties.
Polymers exhibiting high overall gas flux typically possess low chain-chain interactions as is the case with polymers such as poly(dimethylsiloxane) and poly(4-methyl-1-pentene). Due to low chain-chain interactions, these high flux materials have low glass transition temperatures (Tg). Consequently, these materials require special processing conditions to build in chemical and physiochemical crosslinking if they are to be used in high temperature applications. In contrast, polymers having strong chain-chain interactions typically possess rather high Tg values and usually exhibit rather low gas flux.
U.S. Pat. Nos. 3,822,202 and 3,899,309; Re 30,351 (1980), disclose a process for separating fluids using a semi-permeable membrane made from polyimides, polyesters or polyamides. The repeating units of the main polymer chain of these membranes have at least one rigid divalent subunit, the two main chain single bonds extending therefrom which are not colinear wherein at least one of the subunits is sterically unable to rotate 360.degree. around such bond. The repeating units also have 50% or more of its main chain atoms as members of aromatic rings.
U.S. Pat. Nos. 4,705,540 discloses highly permeable aromatic polyimide gas separation membranes and processes for using such membranes. The membranes comprise an aromatic polyimide in which the phenylenediamine groups are rigid and are substituted on essentially all of the positions ortho to the amine functionality and the acid anhydride groups are essentially all attached to rigid aromatic moieties.
U.S. Pat. Nos. 4,717,393 and 4,717,394 disclose polymeric membranes and processes using such membranes for separating components of a gas mixture. The membranes are semi-flexible, aromatic polyimides, prepared by polycondensation of dianhydrides with phenylenediamines having alkyl substituents on all positions ortho to the amine functionality, or with mixtures of other non-alkylated diamines, some components having substituents on all positions ortho to the amine functionality. The membranes formed from this class of polyimides are stated to exhibit improved environmental stability and gas permeability.
U.S. Pat. No. 4,378,400 discloses gas separation membranes formed from aromatic polyimides based upon biphenyltatracarboxylic dianhydride for separating various gas mixtures. Japanese Patent Applications Nos. 1-194904 and 1-194905 disclose gas separation membranes formed from various polyarylates and polyimides, respectively.
Japanese Patent Applications Nos. 63-190507 and 63-278524 disclose gas separation membranes formed from polyamides characterized as having hydrogen atoms on all ring positions ortho to an aromatic amine functionality or, alternatively, as having a maximum of only one alkyl group located on a position ortho to each amine functionality. Additionally, the teachings of both applications are limited to ortho-alkylated compounds wherein the alkyl group is either methyl or ethyl.
M. Salame, Poly. Eng. Sci., 26 1543 (1986), reports a predictive relationship between oxygen permeability coefficient [(PO.sub.2)] and polymer structure wherein the group contributions of various structural portions of a polymer are equated to P(O.sub.2) values. The presence of an aromatic group, such as phenyl, in place of a methylene (--CH.sub.2 --) group is stated to decrease the P(O.sub.2) values for a pair of comparative polymers.